A. Field of the Invention
The present invention relates to a method and an apparatus for detecting the position and the orientation of an interventional device, using a Faraday sensor.
B. Description of the Related Art
Modern interventional and surgical procedures aim to treat the affected tissue by causing minimal injury to healthy tissue structures. Special instruments (e.g., a catheter, an endoscope, or a laparoscope) are delivered to the target region via a small-sized (minimal) surgical opening and the affected tissue is treated.
The target of the minimally invasive operation is often not directly visible for an interventionist (i.e., a surgeon), thus the operations are done under imaging guidance. X-ray fluoroscopy, computed tomography, ultrasound and optical endoscopy are currently the most frequently used imaging methods. However, magnetic resonance imaging (MRI) is a state of the art imaging modality for medical diagnostics, and it has the potential to become an excellent guidance method. Knowing the accurate position of the instrument tip inside the patient is essential, since the surgeon has to be able to accurately guide the instrument to the target.
Within an MR-imaging apparatus, interventional device tracking has so far been done cither by evaluating the change (gain or loss) of the MR-signal at the position of the interventional device or by attaching an electrical conductor or a high-frequency antenna to the device.
The change of the MR-signal caused by the device itself can only be used to measure the position of the device, if the device is positioned within the imaging frame. Furthermore, if the contrast between the device and its surroundings is too low, it cannot be detected. Visualization of an interventional device could depend upon its being coated with an MR contrast agent, or upon its effect on the MR image by nature of its chemical make-up. However, compounds and materials considered MR-compatible, and even MR contrast agents, can produce significant distortion artifacts that obscure the anatomy and physiology of the imaged tissue. Initial attempts to visualize endovascular devices in MR imaging were based on passive susceptibility artifacts produced by the device when exposed to an MR field. U.S. Pat. No. 5,154,179 and U.S. Pat. No. 4,989,608 disclose the incorporation of paramagnetic material into endovascular devices to make the devices visible under MR imaging.
If the device is connected to an electrical conductor with one or more windings and if these windings are passed by a direct current, a local distortion of the magnetic field of the MR-imaging apparatus is caused. This distortion causes a local alteration of the MR image. By taking two MR images, one with and one without a direct current flowing through the conductor, the position of the interventional device can be determined by the difference of the signals in the two MR images. This method is described in U.S. Pat. No. 5,868,674. Within the magnetic radio frequency field of the MR-imaging apparatus, electric coupling of the wires, which are used as a current supply for the electrical conductor, can lead to severe heating under certain circumstances. If the heat is not dissipated, it might result in a malfunction or even in the destruction of the device, which could be hazardous to the patient.
Another possibility for measuring the position of the interventional device is the application of a high-frequency antenna, which is attached to the device. This antenna is tuned to the resonance frequency of the MR-imaging apparatus. Therefore, the antenna is not compatible to all MR imagers. Furthermore, dangerous RF-heating can take place which can be detrimental to the patient, and the MR technique has to be adapted to this kind of device tracking.
Exemplary of methods for active MR visualization of implanted medical devices is U.S. Pat. No. 5,211,165, which discloses an MR tracking system for a catheter based on transmit/receive microcoils positioned near the end of the catheter by which the position of the device can be tracked and localized. U.S. Pat. No. 5,375,596 discloses a method for locating catheters and other tubular medical devices implanted in the human body using an integrated system of wire transmitters and receivers. U.S. Pat. No. 4,572,198 discloses the use of conductive elements, such as electrode wires, for systematically disturbing the magnetic field in a defined portion of a catheter to yield increased MR visibility of that region of the catheter. However, the presence of conductive elements in the catheter also introduces increased electronic noise and the possibility of Ohmic heating, and these factors have the overall effect of degrading the quality of the MR image and raising concerns about the patient's safety. U.S. Pat. No. 5,882,305 therefore replaces long lead wires, which can cause heating during MR imaging and may distort an MR image, by at least one optical fiber. Consequently, electrical signals have to be converted into modulated optical signals and vice versa by two transducer circuits.
Another possible way of finding the position of an interventional device is to localize it independently of the MR imaging, by using several cameras and light-emitting or reflecting reference marks. For this technique, a free field of view between the reference marks and the cameras is required. Unfortunately, the field of view is very limited when the interventional device is inside a patient's body and when the MR-imaging apparatus is closed.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.